Bitcoin-Core, Linux, is not listening on port 8333. How do you fix that?
bitcoin.conf has
port=8333
#asknostr
Discussion
Not listening on loopback or not listening on LAN?
Not listening on any device. 
I have noticed it only works as a startup command line parameters rather than bitcoin.conf
Ah. Do you really need it? Looks like it's for nodes to communicate to one another so it seems crucial
Your node can't contribute to the network unless you expose this port and poke a hole in your firewall (unless you use TOR). If you don't do this then you are just a consumer. It depends on your intent. It's also more secure to have more nodes connected to you. For example, if you are limited to 8 connections and all 8 are bad actors, then you could run into problems (however this is quite rare)
You might want to add to bitcoin.conf:
listen=1
server=1
Thanks, both were already set. Do you need the Initial Sync to be done before 8333 opens?
No, you should be able to connect to it right away. The daemon will know if the initial sync is complete or not and report that accordingly. If you connecting from a different machine you'll need to open up access so rather than 127.0.0.1 you'll need to bind to the ethernet adapter or you could use 0.0.0.0 for all
Also, after making changes to the config, you need to restart bitcoind or bitcoin-qt whichever you're running to re-read to new config